When manufacturing infrared focal plane array detectors, one of the important considerations is the construction of the metal-insulator-semiconductor (MIS) structure. In the past, the standard process for fabricating mercury cadmium tellurium (HgCdTe) focal plane array MIS devices utilized aluminum as the gate metal. Many problems have been encountered when using aluminum as the gate metal for MIS structures. Specifically, the aluminum has suffered severe corrosion when exposed to the chemicals required in subsequent processes. Additionally, the plasma species used to dry etch the aluminum tends to create a corrosive environment, which damages the detector. As a result of these complexities, the uniformity across the entire substrate has suffered, resulting in poor device performance.
One of the primary concerns with aluminum is its significant etching by the bromine solution which is used to form vias or contacts in overlaying zinc sulfide insulator films. As a result, extreme care must be taken not to overetch the zinc sulfide insulator in the bromine solution when forming the vias or contacts to the aluminum substrate.
In order to prevent etching of the aluminum substrate in the bromine solution, additional processing steps must be introduced to form a protective oxide coating on the surface of the aluminum. The oxide coating is not readily etched in the bromine solution; however, additional processing steps must be introduced later to remove the oxide coating in order to make good electrical contact to the aluminum gate.
Another problem associated with the use of aluminum as the metal layer is that the chlorine-containing plasma commonly used to dry etch aluminum films tends to corrode the aluminum films when the metal pattern is subsequently exposed to the ambient atmosphere. As in the first problem discussed above, additional processing steps must be taken to remove any chlorine-containing residue from the etched aluminum film prior to exposure to the atmosphere. These additional processes are burdensome in high volume production.
A need has therefore arisen for a method of forming a metal pattern in an infrared focal plane array detector which avoids corrosion of the metal pattern in the subsequent processes necessary for fabricating the infrared detector and which is compatible with the high volume production of infrared focal plane array detectors.